Machine part

ABSTRACT

Machine part is constituted with steel with the carbon content of 0.2% or more and reduced with the hydrogen content after hardening by a heat treatment to 0.04 ppm or less. Further, hardness after hardening by the heat treatment is made to Hv 450 or more.  
     Since such a machine part is excellent in the super long life fatigue characteristics, it can be used suitably as a bearing ring or a rolling element of a rolling bearing.

TECHNICAL FIELD

[0001] This invention relates to a machine part of excellent super longlife fatigue characteristics and, more in particular, it relates to amachine part used suitably for those portions undergoing repetitive highstresses (such as bending, tension and compression), for example, axlesof vehicles such as bullet trains, gears, blades of turbines, reductiongears such as of automobiles and industrial machines, and bearings.

BACKGROUND ART

[0002] Heretofore, when fatigue fractures by repetitive stresses (suchas the bending stresses, tensile stresses and compressive stresses) donot occur over the number of repetitive cycles N of 10⁷ on metalmaterials such as high strength steels, it is considered that thefatigue fractures do not occur thereafter. Therefore, fatigue limits(the fatigue strength) are determined at the number of repetitivecycles: N=10⁷ cycles.

[0003] In recent years, however, a phenomenon that fatigue fractures donot occur up to 10⁷ cycles but fatigue fractures occur in excess ofnumber of repetitive cycles N of 10⁷ (the phenomenon is referredhereinafter as super long life fatigue fracture and the fatiguecharacteristic the number of at repetitive cycles : N=10⁷ over isreferred to as super long life fatigue characteristic) was reported byNaito, et al (Materials, 32, 361 (1983)) and Emura, et al (KIRON A-55,509 (1989)), which has now attracted attention.

[0004] Since machine parts, for example, axles of vehicles such asbullet trains and blades of turbine are sometimes used under repetitivestresses in excess of 10⁷ cycles, it is important to analyze the causefor the occurrence of the super long life fatigue fracture.

[0005] On the other hand, adverse effects of hydrogen on the staticstrength of high strength steels are well known as the phenomena such asdelayed fracture. However, the effects of hydrogen have been pointed outas factors to reduce the fatigue characteristics of high strengthsteels, for the first time, by Murakami, et al only recently (TheSociety of Materials Science, Japan 24th Fatigue Symposium Proceedings(1998), and Materials, 48.10 (1999)).

[0006] As the method for preventing the deterioration of delayedfracture characteristic there are disclosed, for example, a method oftrapping hydrogen intruding in steels thereby restricting the number ofsulfide compounds and inclusions that form concentration sources ofhydrogen (Japanese Published Unexamined Patent Application No.1746/1998) or a method of dispersing and precipitating fine carbides,nitrides, sulfides in steels to trap intruded hydrogen in a dispersedmanner thereby suppressing hydrogen embrittlement (Japanese PublishedUnexamined Patent Application No. 110247/1998). Such examples concernmainly for steel materials used in applications where a relatively greatamount of hydrogen may possibly intrude from the surface of the steelmaterials (diffusive hydrogen) during use.

[0007] Also, Japanese Published Unexamined Patent Application No.256274/1999 discloses high strength fine steel wires intended for theimprovement of delayed fracture characteristic by restricting the amountof hydrogen intruding in the steel such that the amount of hydrogenreleased upon heating from a room temperature to 300° C. is 0.5 ppm orless.

[0008] However, none of the methods disclosed in Japanese PublishedUnexamined Patent Application No. 1746/1998 and Japanese PublishedUnexamined Patent Application No. 256274/1999 is intended for theimprovement of the super long life fatigue characteristic but for theimprovement of usual fatigue characteristic (delayed fracture) .Further, they are not based on the data that distinctly analyze themechanism of hydrogen on the fatigue characteristic. Accordingly, evenwhen the method described above is used, it is difficult to sufficientlyimprove the super long life fatigue characteristic.

[0009] In view of the above, this invention intends to solve the problemin the prior art as described above and has a subject of providing amachine part of excellent super long life fatigue characteristic.

DISCLOSURE OF THE INVENTION

[0010] In order to solve the subject described above, this inventioncomprises the following constitution. That is, the machine partaccording to this invention is characterized in that it is constitutedwith steel having a carbon content of 0.2% or more and the hydrogencontent after hardening by heat treatment is 0.04 ppm or less.

[0011] Further, it is preferred the surface of the machine part is madeto hardness of Hv 450 or more by a method, for example, of applyingsurface hardening treatment.

[0012] In accordance with the constitution, the machine part describedabove has extremely high reliability since there is less possibility ofcausing undetermined deterioration of fatigue strength due to the effectof hydrogen and the super long life fatigue characteristics areexcellent.

[0013] Accordingly, the machine part according to this invention can beapplied suitably to various machine parts which are used underrepetitive stresses of over 10⁷ cycles by rotations or vibrations, suchas bearing rings or rolling elements of rolling bearings.

[0014] The present inventors have accomplished this invention based onthe following findings obtained as a result of extensive studies for theeffects of inclusions and hydrogen on the super long life fatiguefracture of high strength steels.

[0015] Present inventors have reported that non-metallic inclusions arepresent at the fracture starting points of steel test specimenssuffering from super long life fatigue fracture, and regions appearingmore dark under metal microscopic observation because of the roughsurface state are present at the periphery of the inclusion (hereinafterreferred to as ODA: Optically Dark Area) and hydrogen has an importantrole to the formation thereof (KIRON A-66, 642 (2000)).

[0016]FIG. 1 shows a scanning electron microscopic (SEM) photographobserving inclusions at the fracture starting point, ODA and thevicinity thereof in a test specimen (made of SCM 435) suffering fromsuper long life fatigue fracture. As shown in FIG. 1, while typicalfatigue fracture surface is served in the martensitic tissue at theoutside of the ODA, the martensitic tissue is not distinctly observed inthe ODA and a tissue that appears more fragile than usual fatiguefracture surface is observed.

[0017] ODAs around the inclusion (refer to FIG. 1) are observed in atest specimen fractured at long life by a low stress fracture test butis not observed in a test specimen fractured at a short life by a highstress fracture test. It is supposed from the foregoings that the ODAswere caused as a result of discontinuous development of cracks by themechanism similar with delayed fracture by hydrogen trapped to theperiphery of the intrusion and the repetitive stresses.

[0018] Then, the following test was conducted in order to investigate arelation between the dimension of the ODA and the hydrogen content. Thatis, a heat treatment (quenching) for the test specimen was conducted ina hydrogen-containing atmosphere (for example, in Rx gas) or in vacuum,and a fatigue test was conducted for each of the test specimens. Whenthe fracture starting points of the test specimens fractured at about anidentical fracture life were observed, the dimension of the ODA presentat the periphery of the inclusion as the fracture starting point wasconsiderably smaller in the specimen applied with quenching afterheating in vacuum than in the test specimen applied with quenching afterheating in the hydrogen-containing atmosphere. From the result describedabove, it has been found that a correlation exists between the dimensionof the ODA and the hydrogen content.

[0019] The present inventors have already proposed an estimated equationfor estimating the fatigue limit depending on the dimension of thenon-metallic inclusion in order to evaluate the effect of the dimensionof the non-metallic inclusion (defect) on the fatigue strength (Metalfatigue: Micro-defect and Inclusion, 1993, Yokendo), and it is possibleto forecast the fatigue strength of a member according to the estimatedequation. The estimated equation uses the dimension of the non-metallicinclusion, that is, the square root for the area of the non-metallicinclusion (hereinafter referred to as {square root}{square root over ()}area) as a parameter.

[0020] The present inventors have found that the fatigue fracture occurswhen the dimension of the non-metallic inclusion ({square root}{squareroot over ( )}area) and the dimension of the sum for the non-metallicinclusion and the ODA (square root of the sum for the area of thenon-metallic inclusion and the area of the ODA; hereinafter referred toas {square root}{square root over ( )}area′) exceed the limit valuedefined by the estimated equation.

[0021] In other words, they have found that a longer life can beexpected when the dimension for the sum of the non-metallic inclusionand the ODA ({square root}{square root over ( )}area′) is made smallerby the reduction of hydrogen content. As the measure for indicating thedimension of the non-metallic inclusion and ODA, ({square root}{squareroot over ( )}area′)/({square root}{square root over ( )}area) is usedpreferably.

[0022] In addition, the present inventors have found that there is atendency that the dimension of the ODA is larger (that is, the value:({square root}{square root over ( )}area′)/({square root}{square rootover ( )}area) is larger) as the fatigue life is longer in a case wherethe hydrogen content exceeds 0.04 ppm. That is, the fatigue fractureoccurs when the value of ({square root}{square root over ()}area′)/({square root}{square root over ( )}area) is larger. From theforegoings, for longer life, it is desirable that the value of ({squareroot}{square root over ( )}area′)/({square root}{square root over ()}area) at the repetitive number of cycles : N=5.0×10⁷ is 1.5 or less.

[0023] Further, the restriction on the hydrogen content described aboveis particularly effective in a case where the carbon content in thesteel constituting the machine part undergoing the repetitive stressesis 0.2% or more and the hardness is Hv 450 or more.

[0024] That is, the dimension of the ODA can be decreased when thecarbon content is 0.2% or more, the hardness is Hv 450 or more and,further, the hydrogen content is 0.04 ppm or less in the steelconstituting the machine parts undergoing the repetitive stresses.

[0025] Accordingly, they undergo fatigue below the limit value for theestimated value of the fatigue limit calculated by the estimatedequation in view of the dimension of the defect (non-metallic inclusion)and longer life can be attained. Also, when the dimension of thenon-metallic inclusion is restricted in the step of designing themachine part, the fatigue limit of the machine part can be estimated atlong life and a machine part of high reliability excellent in the superlong life fatigue characteristic can be expected.

[0026] Then, it is possible to forecast, in the stage of design, thoseadaptable to various machine parts such as gears, bearings, turbines andaxles to be used frequently while undergoing repetitive stresses over10⁷ cycles by rotations and vibrations.

[0027] The machine parts in accordance with the invention means thoseparts constituting equipments such as prime movers and operationmachines that convert the energy supplied from outside to specifieduseful jobs. For example, they mean axles of vehicles such as bullettrains, gears, turbines, and pumps. In addition, they can be applied toouter rings, inner rings, and cages constituting rolling bearings, aswell as rolling elements constituting rolling devices such as linearguide bearings and ball screws.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a view illustrating an SEM photograph observing afracture surface of a test specimen suffering from super long lifefatigue fracture.

[0029]FIG. 2 is a view for explaining the shape and the dimension of atest specimen used in a fatigue test.

[0030]FIG. 3 is a graph illustrating a relation between the number ofrepetitive cycles and the value of ({square root}{square root over ()}area′)/({square root}{square root over ( )}area) in the fatigue test.

[0031]FIG. 4 is a graph illustrating a relation between the hardness Hvof a test specimen and the value of ({square root}{square root over ()}area′)/({square root}{square root over ( )}area).

BEST MODE FOR EMBODYING THE INVENTION

[0032] An embodiment of a machine part according to this invention willbe described with reference to the drawings. This embodiment shows anexample of this invention, and the invention is not limited to theembodiment.

[0033] Various test specimens were manufactured, and the results ofconducting a fatigue test will be described.

[0034] As the steel materials for the test specimens, SUJ2 (carboncontent: 0.97%), SCM435 (carbon content: 0.35%), and SCR420 (carboncontent: 0.2%) were used, and test specimens of the shape and dimensionas shown in FIG. 2 were manufactured.

[0035] The heat treatment on the test specimens was conducted under theconditions of applying quenching at 840° C. for 30 min and then applyingtempering at 180° C. for 120 min. In this case, the test specimens weremanufactured while varying the hydrogen content by conducting heating ina hydrogen-containing atmosphere (Rx gas containing about 30% hydrogen)or in vacuum and then applying quenching. Then, the effect of thehydrogen content on the fatigue characteristic was examined. In thedescription to follow, the heat treatment of applying heating in thehydrogen-containing atmosphere is referred to as QT heat treatment andthe heat treatment of applying heating in vacuum is referred to as VQheat treatment.

[0036] However, heating may be conducted in a gas not containinghydrogen (for example, an inert gas such as nitrogen) instead ofconducting heating in vacuum for keeping the hydrogen content low in thetest specimen. Further, similar effect can be obtained also by applyingtempering at a slightly higher temperature after usual quenching.

[0037] The test specimens applied with the heat treatment weresurface-finished to #2000 by polishing using Emery paper. Then, aftermeasuring the hydrogen content in the steels, they were put to a fatiguetest.

[0038] The hydrogen content in the steels (machine parts after hardeningby heat treatment) was measured by a temperature elevating hydrogenanalysis method. The measuring method for the hydrogen content by thetemperature elevating hydrogen analysis method is to be detailed below.

[0039] A specimen was inserted in a glass tube and the temperature waselevated at a temperature elevation rate of 15° C./min by using aninfrared ray image furnace for heating with infrared rays from theoutside. Then, hydrogen released from the test specimen in a temperaterange from a room temperature to 800° C. was introduced to a massspectrometer (MASSMATE 200, manufactured by Nippon Shinku Gijutsu Co.)to measure the amount of hydrogen. The temperature was measured bycontacting thermocouples with the test specimen.

[0040] In the mass spectrometer, the hydrogen gas released in vacuum isionized in a mass spectrometer tube. The ionization intensity isconverted to a hydrogen releasing rate based on the ionization intensityof a calibrated standard leak. The hydrogen concentration is determinedby integrating the hydrogen releasing rate. The profile of the hydrogenreleasing rate was compared between a material charged with hydrogen anda material not charged with hydrogen to decide whether hydrogen wasreleased or not and, further, positions stabilized at the lowest levelsbefore and after thereof were connected to determine as the background.

[0041] The fatigue test is a tensile and compression fatigue test with astress ratio of R=−1 (meaning that the tensile stress is equal with thecompressive stress) and at a repeating rate of 35-42 Hz. In the tensileand compressive fatigue test, a slightly lower fatigue strength tends tobe obtained when a bending stress exerts on the test specimen.Accordingly, strain gages are attached at positions for equally dividingthe circumference into four portions near a grip part of the testspecimen and the test specimen was attached to a fatigue tester with acareful adjustment so as not to exert the bending stress on the testedpart upon loading of a stress. However, similar effects can also beobtained qualitatively even when bending stresses or shearing stressesare applied repetitively.

[0042] The maximum contact pressure determined based on the sum for thepermanent deformation amount of a bearing ring and a rolling element ofa rolling bearing (0.0001 times of the diameter of rolling element) is3.92 GPa (400 kgf/mm²). They are used actually in a state where 2.94 GPz(300 kgf/mm²) of stresses are loaded repetitively (refer, for example,to Japanese Published Unexamined Patent Application No. 61372/1996).

[0043] Application of this invention excellent in the super long lifefatigue characteristics to machine parts loaded with such large stressesrepetitively is effective for the improvement of the fatigue life andthe improvement of the reliability of the machine part.

[0044] The fracture surface of the test specimen fractured in thefatigue test was observed by a scanning electron microscope (JSM-T 220A,manufactured by Nippon Electron Co. Ltd.), to examine the dimension of anon-metallic inclusion which is the starting point of fracture ({squareroot}{square root over ( )}area), and the dimension of a portion of thesum for the non-metallic inclusion and the ODA at the periphery thereof({square root}{square root over ( )}area′). Then, the value: ({squareroot}{square root over ( )}area′)/({square root}{square root over ()}area) was determined.

[0045] That is, projection areas of the non-metallic inclusion and theODA were determined by utilizing the difference of contrast (refer toFIG. 1) from two-dimensional scan images of SEM, and the value: ({squareroot}{square root over ( )}area′)/({square root}{square root over ()}area) was calculated.

[0046] The results are shown in Tables 1 and 2. Each table shows,species of steels for test specimens, content of hydrogen, hardness Hv,repetitive stresses loaded in the fatigue test, repetitive number ofcycles until fracture in the fatigue test, dimension of non-metallicinclusion ({square root}{square root over ( )}area) by SEM observation,and value: ({square root}{square root over ( )}area′)/({squareroot}{square root over ( )}area) (dimension ratio), successively fromthe left of the table. Further, all of the test specimens in exampleswere applied with VQ heat treatment, the test specimens of ComparativeExamples 1-4 were applied with VQ heat treatment, and for the rest ofthe examples, QT heat treatment was applied. TABLE 1 Hydrogen amountHardness Stress Number of Dimension of Dimensional Steel species ppm HvMPa Repetition inclusion¹⁾ ratio²⁾ Example 1 SCM435 0.01 561 702 5.83 ×10⁶ 35.4 1.21 Example 2 SCM435 0.01 561 600 1.43 × 10⁷ 55.6 1.18 Example3 SCM435 0.01 561 540 4.80 × 10⁸ 22.3 1.39 Example 4 SCM435 0.01 561 6405.30 × 10⁷ 22.3 1.39 Example 5 SCR420 0.04 450 710 1.02 × 10⁶ 29.6 1.12Example 6 SCR420 0.04 450 600 8.84 × 10⁷ 24.0 1.52 Example 7 SCR420 0.04450 540 9.01 × 10⁷ 18.7 1.33 Comp. Example 1 SUJ2 0.07 700 855 2.38 ×10⁵ 82.0 1.02 Comp. Example 2 SUJ2 0.07 700 857 3.25 × 10⁶ 23.4 1.78Comp. Example 3 SUJ2 0.07 700 819 5.23 × 10⁶ 44.3 1.40 Comp. Example 4SUJ2 0.07 700 799 4.55 × 10⁷ 23.9 2.05

[0047] TABLE 2 Hydrogen amount Hardness Stress Number of Dimension ofDimensional Steel species ppm Hv MPa Repetition inclusion¹⁾ ratio²⁾Comp. Example 5 SUJ2 0.8 700 884 4.50 × 10⁵ 72.3 1.22 Comp. Example 6SUJ2 0.8 700 853 6.98 × 10⁶ 23.7 2.15 Comp. Example 7 SUJ2 0.8 700 8312.65 × 10⁷ 21.3 2.20 Comp. Example 8 SUJ2 0.8 700 763 4.41 × 10⁷ 32.32.48 Comp. Example 9 SUJ2 0.8 700 840 5.40 × 10⁷ 18.0 2.70 Comp. Example10 SCM435 0.8 561 821 1.11 × 10⁶ 22.2 1.37 Comp. Example 11 SCM435 0.8561 781 6.54 × 10⁶ 18.7 1.78 Comp. Example 12 SCM435 0.8 561 641 3.44 ×10⁷ 20.4 2.40 Comp. Example 13 SCM435 0.8 561 601 4.39 × 10⁷ 21.0 2.25Comp. Example 14 SCM435 0.8 561 510 9.70 × 10⁷ 19.0 2.43 Comp. Example15 SCR420 0.2 450 800 2.93 × 10⁶ 20.8 1.55 Comp. Example 16 SCR420 0.2450 740 3.90 × 10⁷ 19.4 2.10 Comp. Example 17 SCR420 0.2 450 550 9.80 ×10⁷ 19.8 2.40

[0048] Then, among the results described above, FIG. 3 shows a graphtaking the value: ({square root}{square root over ( )}area′)/({squareroot}{square root over ( )}area) on the ordinate and the repetitivenumber on the abscissa.

[0049] As a result, it can be seen that, in the examples with thehydrogen content of 0.04 ppm or less, the dimension of the ODA is small(that is, the value: ({square root}{square root over ( )}area′)/({squareroot}{square root over ( )}area) is as small as 2.05 or less) even whenthe number of repetition N exceeds 10⁷ cycles and, the fatigue cracksdue to hydrogen around the inclusion as the center less develop. On theother hand, in comparative examples with the hydrogen content in excessof 0.04 ppm, the value ({square root}{square root over ()}area′)/({square root}{square root over ( )}area) exceeds 2.05 when therepetition number N exceeds 10⁷ cycles and it can be seen that the ODAincreases by the development of fatigue cracks.

[0050] Since the fatigue strength is lower as the inclusion (defect) asthe fracture starting point is larger, it is desirable that thedimension of the defect as the fracture starting point is smaller inorder to attain the excellent super long life fatigue characteristic. Asdescribed above, when the hydrogen content is restricted to 0.04 ppm orless, since extension of the defect due to hydrogen is small (since theODA less extends by the development of fatigue cracks), excellent superlong life fatigue characteristic can be provided to the steel material.

[0051] Then, the hardness for each test specimen (carbon content) isnoted and the result of the study on the relation with the super longlife fatigue characteristic is to be described.

[0052] For the steel material of the test specimens, SUJ2 (carboncontent: 0.97%), SCM435 (carbon content: 0.35%), SCR420 (carbon content:0.2%) and SCM415 (carbon content: 0.15%) were used and the testspecimens each of the shape and the dimension shown in FIG. 2 weremanufactured.

[0053] The heat treatment for the test specimens were identical with thecase described above, and test specimens with varied hydrogen contentfor each kind of the steel materials respectively (hydrogen content:0.04, 0.08, 0.3 ppm) were manufactured by applying either the QT heattreatment or the VQ heat treatment.

[0054] When the hardness Hv was measured for each test specimen, thehardness Hv was higher as the carbon content was increased. That is, thehardness for the test specimen was Hv 720 for those made of SUJ2, Hv 560for those made of SCM435, Hv 450 for those made of SCR420 and Hv 400 forthose made of SCM415.

[0055] Successively, the same fatigue test as described above wasconducted, and for the test specimens fractured at the repetition numberN: 3×10⁷−5×10⁷ cycles, the fracture surface was measured by SEM. Then,in the same manner as described above, the dimension ({squareroot}{square root over ( )}area) for the non-metallic inclusion as thefracture starting point and the dimension ({square root}{square rootover ( )}area′) for the portion of the non-metallic inclusion and theODA at the periphery thereof were examined. Then, the value: ({squareroot}{square root over ( )}area′)/({square root}{square root over ()}area) was determined.

[0056] Among the results, FIG. 4 shows a graph taking the value:({square root}{square root over ( )}area′)/({square root}{square rootover ( )}area) on the ordinate and the hardness Hv on the abscissa.

[0057] As a result, the test specimens with the hydrogen content of 0.04ppm can be expected for excellent super long life fatigue characteristicin which the dimension of the ODA is small (that is, the value: ({squareroot}{square root over ( )}area′)/({square root}{square root over ()}area) is small) for any of the steel species (for any of hardness).

[0058] However, it can be seen, in the test specimens with the hydrogencontent of 0.08 ppm or more, that the region regarded as the defectextends and it was difficult to expect for excellent super long lifefatigue characteristic with the value: ({square root}{square root over ()}area′)/({square root}{square root over ( )}area) being 2 or more,excepting for SCM415 with the hardness of Hv 400.

[0059] For the high strength steels it has been known so far that theytend to undergo the effect of hydrogen content more as the tensilestrength is higher and show deterioration for the fatigue strength in adelayed fracture manner. Since a correlation exists between the tensilestrength and the hardness, this may be attributable to that they undergothe effect of the hydrogen content more as the hardness is higher alsoin the case of this test specimen. Then, it was confirmed that thecritical value is at Hv 450 in view of the graph shown in FIG. 4 andreduction of the hydrogen content is effective to the improvement of thesuper long life fatigue characteristics at Hv 450 or more.

[0060] In a case of SCM415 with the hardness of Hv 450 or lower, thevalue of ({square root}{square root over ( )}area′)/({squareroot}{square root over ( )}area) is not so large even when the hydrogencontent exceeds 0.04 ppm (no correlation between the hydrogen contentand the value of ({square root}{square root over ( )}area′)/({squareroot}{square root over ( )}area)), and excellent super long life fatiguecharacteristic can be expected. Accordingly, in the invention ofreducing the hydrogen content for improving the super long life fatiguecharacteristic, it is necessary that the steel hardness is Hv 450 ormore.

[0061] Since the carbon content in the steel has to be 0.2% or more inorder to obtain a hardness of Hv 450 or more, it is necessary that thecarbon content in the steel is 0.2% or more in this invention.

[0062] As has been described above, since the steels with the hydrogencontent of 0.04 ppm or less and the carbon content of 0.2% or more causeless hydrogen-induced extension of defects, it is excellent in the superlong life fatigue characteristic. Accordingly, such steel materials canbe applied suitably to machine parts that are used sometimes underrepetitive stresses exceeding 10⁷ cycles by rotations or vibration.

[0063] Further, they are also applicable to outer rings, inner rings,rolling elements and cages constituting rolling bearings, as well asrolling members constituting rolling devices such as linear guidebearings and ball screws.

Industrial Applicability

[0064] As has been described above, since the machine part applied withthis invention is excellent in the super long life fatiguecharacteristics, it is highly reliable.

1. A machine part constituted with steel with the carbon content of 0.2%or more, in which the hydrogen content after hardening by heat treatmentis 0.04 ppm or less.
 2. A machine part as defined in claim 1, whereinthe hardness after hardening by heat treatment is Hv 450 or more.
 3. Amachine part as defined in claim 1 or 2, which is used as a bearing ringor a rolling element of a rolling bearing.